As is known, in certain operating conditions, internal combustion engines are subject to detonation, i.e. uncontrolled combustion of the mixture, which, when severe and occurring in numerous engine cycles, results in impaired efficiency and serious overheating of the engine, as well as reduced working life and abrupt failure of certain parts of the engine.
For this reason, several systems have been proposed or are currently being developed for detecting detonation conditions or the phenomenon itself, and so reducing the likelihood or consequences of detonation by controlling combustion parameters accordingly.
Such systems are mainly based on monitoring, directly or indirectly, the pressure as a function of piston position inside the cylinder, the cycle graph of which is bell-shaped with the peak close to the top dead center position. Under normal combustion conditions, the peak is typically rounded, whereas, in the event of detonation, it comprises a jagged edge with a large number of indentations (FIGS. 1a and 1b).
Analysis of these indentations provides useful information for the purpose of detonation detection.
Some known methods employ sensors located inside the combustion chamber to detect the amplitude of the indentations directly. Though enabling highly accurate, reliable values to be obtained, such an arrangement of the sensors involves sophisticated high-cost technology, and is therefore restricted to laboratory use or prototypes.
Other methods employ vibration sensors fitted to the engine block, which, though technically simpler and cheaper, involves greater disturbance of the readings as compared with direct direction, due to the vibration measured on the block also being caused by other phenomena in addition to the block-filtered variation in pressure inside the cylinder.
Patent GB-A-2 265 006, filed by Nippondenso Co. Ltd, relates, for example, to a detonation control system employing detonation sensors on the engine block, and which detects detonation by comparing the intensity of the sensor signal with a decision threshold, and eliminates detonation by controlling engine parameters, such as injection advance. More specifically, the system performs a logarithmic conversion of the intensity of the sensor signal, works out its distribution, calculates a value corresponding to the standard deviation of said distribution, and compares said value with a threshold calculated on the basis of the previously calculated value and a mean value of said distribution.
Distribution is determined by processing numeric values representing the amplitude of spectral components obtained by narrow-band filtration of the output signal of the sensors; which signal is composed of numerous harmonics, and is narrow-band filtered to only extract the harmonic with the highest energy content.
In the above patent, the mean value and standard deviation are calculated with reference to one engine cycle at a time, and by adapting the detonation threshold at each cycle.
In certain cases, however, controlling operation of the engine on the basis of one spectrum frequency and one engine cycle at a time is restrictive.
That is, concentrating on one frequency may result in others, also energized by detonation, being disregarded; and controlling the engine by simply eliminating local detonation cycle by cycle may not always be the best solution, in the sense that engine efficiency is at times better promoted by allowing a given number of detonations every so many engine cycles and/or in only certain cylinders. Trace detonation, in fact, by permitting a high degree of efficiency, is a favourable engine operating condition and therefore one to be exploited.
Finally, adapting the decision threshold involves extensive numeric processing, which further complicates implementation of the method.